With the general recognition of the existence of finite limitations to the availability of petrochemical and other sources of fuel, much attention has been directed to the utilization of solar energy for heating, cooling and other purposes. The patent literature contains numerous references to specific systems, while the scientific literature, which is now much greater in volume, has largely been concerned with the more conceptual and theoretical aspects of such systems but now evidences many experimental installations. A recent article entitled "Solar Heating and Cooling," by J. A. Duffie and W. A. Beckman, published in Science, pp. 143-149, Volume 191, Number 4223, Jan. 16, 1976, evidences the current state of the art in many respects and provides a useful bibliography. The same authors have also published a text, "Solar Energy Thermal Processes" (Wiley, New York, 1974) which is more comprehensive in nature and widely employed by workers in the art.
In the referenced article, Duffie and Beckman discuss the solar air heater and solar water heater systems which are principally used for collection, and the thermal energy storage systems (pebble bed and water storage tank respectively) which are usually employed with them, and point out that the water heater type of system is predominantly used. The water heater system can of course present freezing and boiling problems; because of the need for piping to conduct the liquid and for a good thermal interchange between the collector and the liquid in the conduit, collectors for solar water heater systems are substantially more expensive than those for air heater systems. Both types of systems use radiant energy transmissive cover panels, generally referred to as glass covers, and an insulative structure at the back and edges of the plates. As indicated in the article solar energy collection can also be used in a cooling system, although an additional thermodynamic process is needed.
Beckman and Duffie give a generalized equation often used to define the useful gain of a collector, which is determined by flow rate, temperature gradients, absorbed radiation, angle of incidence of the solar energy, the number of covers, the properties of the collector plate and the covers, and a number of other factors. They point out that an increase in collector temperature causes an increase in thermal loss that diminishes collector output, as it approaches the absorbed radiation. They do not discuss another very significant element of thermal loss which arises from natural (free) convection losses caused by the heating of the light incident upper side of the collector plate. More than 70 years ago it was shown that cellular recirculation of gases on the surface of the collector plate could arise, substantially intensifying convection losses should this condition exist. These so-called "cells of Benard" have subsequently been studied in much more detail, and various schemes have been devised for minimizing or suppressing this effect. The proposed techniques include the use of an open honeycomb type of structure and multiple planar absorber elements geometrically arranged to inhibit generation of the cellular action. Such expedients, however, materially increase collector panel costs and introduce other, if relatively lesser, losses in the system.
A number of other expedients for improving collector efficiency are mentioned in the referenced article, including the usage of selective, low emissivity, surfaces and the employment of a vacuum in at least one interior volume within the collector structure. The former expedient may advantageously be used with most collector systems, including the presently disclosed system; the latter expedient presents economic and maintenance penalties that should be avoided if at all possible.
The factors represented in the thermal mass calculations given by Duffie and Beckman do not include a term for the significance of the thermal mass of the system, but it is obvious that the time delays encountered in heating the collector system (or what may be called the thermal capacitance of the system) can be of primary importance in environments in which the incident solar radiation is weak, intermittent or both. Thus in a geographical area at high latitude the thermal capacitance of a solar water heater may not be acceptable. Typically, for such a system, the thermal mass of the collector plate requires approximately 5 minutes for heating to a 50.degree. C. temperature differential above ambient, and another 15 minutes is required for heating of the water mass. In addition, solar energy systems used at low latitudes are particularly subject to high wind losses and low ambient temperature conditions. The usual approach of adding additional glass covers does decrease heat loss but at the penalty of substantial attenuation and reflection of the radiant energy. It would be far preferable to be able to reduce the thermal capacitance while at the same time minimizing the number of glass covers and avoiding introduction of a sealed vacuum system.